Salesperson returning after six is a snake (5)
I believe the answer is:
viper
'snake' is the definition.
(viper is a kind of snake)
'salesperson returning after six' is the wordplay.
'salesperson' becomes 'rep' (I've seen this before).
'returning' says the letters should be written in reverse.
'after' means one lot of letters go next to another.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'rep' backwards is 'per'.
'per' put after 'vi' is 'VIPER'.
'is a' is the link.
(Other definitions for viper that I've seen before include "poisonous type" , "Deadly-venomous snake" , "Malicious type" , "Adder" , "spiteful person" .)